Untitled Quiz

Cellular Respiration and Photosynthesis

• Cellular respiration and photosynthesis are complementary processes. Photosynthesis captures light energy and converts it into chemical energy in the form of glucose. Cellular respiration breaks down glucose to release energy in the form of ATP, the energy currency of the cell.

• Respiration "breathing" is the exchange of gases between the body and the environment. Cellular respiration is a metabolic process that occurs within the cells of organisms to generate energy.

• Fermentation is anaerobic, meaning it doesn't require oxygen. It produces a small amount of ATP by breaking down glucose into pyruvate and then further converting it to different molecules like lactic acid or ethanol. Aerobic cellular respiration, on the other hand, requires oxygen and generates a much larger amount of ATP.

Summary Equation

• The summary equation for cellular respiration is: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP).

ATP Production

• Approximately 38 ATP molecules are generated from the complete oxidation of one glucose molecule in cellular respiration.

• The human body uses ATP for various cellular processes, including muscle contraction, nerve impulse transmission, protein synthesis, and active transport across cell membranes.

Oxidation and Reduction

• Oxidation is the loss of electrons, often accompanied by a gain of oxygen or loss of hydrogen.

• Reduction is the gain of electrons, often accompanied by a loss of oxygen or gain of hydrogen.

• Redox reactions involve the transfer of electrons from one molecule to another, releasing energy stored in the chemical bonds.

The Role of NAD+^

• NAD+ is a coenzyme that acts as an electron carrier in cellular respiration. It picks up electrons during oxidation and transfers them to the electron transport chain, where energy is released to pump protons across the mitochondrial membrane.

Electron Transport Chain

• The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane. Electrons are passed from one complex to the next, releasing energy at each step. This energy is used to pump protons from the mitochondrial matrix to the intermembrane space.

Stages of Cellular Respiration

• Glycolysis occurs in the cytoplasm and breaks down glucose to pyruvate, generating a small amount of ATP (2 ATP).
• Citric Acid Cycle (Krebs Cycle) takes place in the mitochondrial matrix and oxidizes pyruvate to carbon dioxide, generating additional ATP (2 ATP) and reducing NAD+ and FAD.
• Electron Transport Chain and Oxidative Phosphorylation occur in the inner mitochondrial membrane, where electrons are passed along a series of protein complexes, releasing energy to pump protons, creating a proton gradient. This gradient drives ATP synthesis by ATP synthase.

ATP Production in Each Stage

• Glycolysis: ATP is produced by substrate-level phosphorylation, where a phosphate group is transferred from a substrate molecule directly to ADP.

• Citric Acid Cycle: ATP is produced by substrate-level phosphorylation, similar to glycolysis.

• Electron Transport Chain: ATP is produced by oxidative phosphorylation, where proton movement across the membrane through ATP synthase drives the synthesis of ATP from ADP.

Glycolysis

• Glycolysis requires ATP for its preparatory steps, two ATP molecules are used to activate the glucose molecule.

• Glycolysis is an anaerobic process and can occur in the absence of oxygen.

Pyruvate Oxidation and Acetyl CoA

• Pyruvate is oxidized to acetyl CoA in the mitochondrial matrix.

• This process removes a carbon dioxide molecule from pyruvate, generates NADH, and links glycolysis to the citric acid cycle.

Citric Acid Cycle

• The citric acid cycle produces:

  • NADH and FADH2: These electron carriers transfer electrons to the electron transport chain.
  • ATP: Generated by substrate-level phosphorylation.
  • CO2: Released as a waste product.

• The citric acid cycle is called a cycle because the end product of the cycle (oxaloacetate) is a reactant for the first step.

Substrate-level Phosphorylation vs. Oxidative Phosphorylation

• Substrate-level phosphorylation is the direct transfer of a phosphate group from a substrate molecule to ADP, generating ATP. This occurs during glycolysis and the citric acid cycle.

• Oxidative phosphorylation is the process where ATP is generated by the movement of protons across a membrane, driven by the energy released from electron transport. It is the primary source of ATP in cellular respiration.

Electron Transport Chain and Proton Gradient

• The electron transport chain in the inner mitochondrial membrane uses the energy from electrons to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (higher concentration of protons in the intermembrane space).

The Final Electron Acceptor

• Oxygen is the final electron acceptor of the electron transport chain.

Oxidative Phosphorylation Steps

• Electron Transport: Electrons are passed from one protein complex to the next, releasing energy to pump protons.

• Chemiosmosis: Protons flow down their concentration gradient, through ATP synthase, driving the synthesis of ATP.

ATP Synthase

• ATP Synthase is a molecular machine embedded in the inner mitochondrial membrane, acts as a "proton turbine".

• Protons flow down their concentration gradient through the channel in ATP synthase, driving the rotation of a central stalk. This rotation causes conformational changes in the ATP synthase molecule, generating ATP from ADP and inorganic phosphate.

Proton Gradient in Mitochondria

• The proton (H+) concentration is highest in the intermembrane space of the mitochondria and lowest in the mitochondrial matrix.

ATP Yield

• The total ATP yield from the complete oxidation of one glucose molecule in cellular respiration is approximately 38 ATP. However, this is a theoretical maximum, and the actual yield can vary.

Brown Fat

• Brown fat is a type of fat found in mammals that generates heat instead of ATP. It is important for maintaining body temperature, especially in newborns and during hibernation.

• Brown fat contains high levels of mitochondria and a protein called thermogenin, which uncouples proton movement from ATP synthesis. This means that the energy from the electron transport chain is released as heat instead of being used to produce ATP.

• Brown fat is darker in color than white fat, due to the high concentration of mitochondria and capillaries.

Fermentation

• Fermentation is an anaerobic process that occurs in the absence of oxygen and generates a small amount of ATP. It is important for organisms that live in oxygen-deprived environments.

• Fermentation is different from anaerobic respiration because it does not involve an electron transport chain or oxidative phosphorylation.

• The basic function of fermentation is to regenerate NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen.

Pyruvate in Fermentation

• In alcohol fermentation, pyruvate is converted to ethanol and CO2.
• In lactic acid fermentation, pyruvate is converted to lactic acid.

Fermentation End Products

• The end products of alcohol fermentation are ethanol and carbon dioxide.
• The end products of lactic acid fermentation are lactic acid.

Comparison of Fermentation and Cellular Respiration

• Both fermentation and cellular respiration begin with glycolysis.
• However, fermentation does not have a citric acid cycle or electron transport chain, and it generates much less ATP than cellular respiration.

Anaerobic and Facultative Anaerobes

• Obligate anaerobes are organisms that can only survive in the absence of oxygen and may even be poisoned by oxygen.
• Facultative anaerobes can switch between aerobic respiration (using oxygen) and fermentation depending on the availability of oxygen.

Glycolysis Evolution

• The glycolysis pathway is an ancient metabolic pathway, suggesting that life likely evolved in an oxygen-poor environment.

Connections Between Metabolic Pathways

• Food molecules other than glucose can be oxidized to make ATP. For example, carbohydrates (e.g., starches), proteins, and fats can be broken down into molecules that can enter the glycolysis, citric acid cycle, or electron transport chain at various points.

• Glycolysis and the citric acid cycle can contribute to anabolic pathways: the molecules produced during these catabolic processes can be used as building blocks for the synthesis of larger molecules.

Introduction to Photosynthesis

• Autotrophs are organisms that can produce their own food from inorganic sources, such as sunlight, using photosynthesis. Plants, algae, and some bacteria are autotrophs.

• Heterotrophs are organisms that obtain energy from consuming other organisms. Animals, fungi, and most bacteria are heterotrophs.

Chloroplast Structure

• Chloroplasts are the sites of photosynthesis in eukaryotic cells.
• They have three membrane systems:
  • Outer membrane: Encloses the entire organelle.
  • Inner membrane: Surrounds the stroma.
  • Thylakoid membrane: Forms interconnected flattened sacs called thylakoids, creating a compartment called the thylakoid lumen.

Photosynthesis Summary Equation

• The summary equation for photosynthesis is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.

Redox Reactions in Photosynthesis

• Photosynthesis involves redox reactions where electrons are transferred from water to carbon dioxide. Water is oxidized, losing electrons and releasing oxygen. Carbon dioxide is reduced, gaining electrons and becoming sugar.

Main Stages of Photosynthesis

• Photosynthesis has two main stages:
  • Light reactions: These occur in the thylakoid membrane and use light energy to generate ATP and NADPH.
  • Calvin Cycle: It takes place in the stroma of the chloroplast and uses ATP and NADPH produced in the light reactions to fix carbon dioxide and produce sugar.

Light Reaction Equation

• H2O + light → O2 + ATP + NADPH

Calvin Cycle Equation

• CO2 + ATP + NADPH → sugar

Wavelengths of Visible Light

• Visible light comprises wavelengths between approximately 380 nm (violet) and 750 nm (red).

• Red wavelengths: About 620-750 nm

• Blue wavelengths: About 400-470 nm

• Shorter wavelengths have higher energy, and longer wavelengths have lower energy.

Carotenoids

• Carotenoids are accessory pigments that absorb wavelengths of light that chlorophyll cannot.
• They protect chlorophyll from damage by absorbing excess light energy, which can be harmful to the cell.

Effective Wavelengths for Photosynthesis

• Photosynthesis is most effective in the blue and red regions of the visible light spectrum.

Chlorophyll a Absorption

• Chlorophyll a absorbs photons, causing its electrons to become excited.
• In an intact chloroplast, the excited electrons are passed along an electron transport chain, releasing energy that drives the production of ATP and NADPH.

Photosystems

• Photosystems: Complexes of pigments and proteins involved in capturing light energy.
• Light-harvesting complexes: Antenna pigments that absorb light energy and transfer it to the reaction center.
• Reaction center: A special pair of chlorophyll molecules where light energy is converted into chemical energy.

Electron Flow in Linear Electron Flow

• Linear electron flow involves two photosystems, Photosystem II (PSII) and Photosystem I (PSI).
• PSII: Light energy excites electrons in the reaction center, which are passed along an electron transport chain.
• Water splitting: PSII splits water molecules, releasing oxygen as a byproduct.
• Proton gradient: The electron transport chain pumps protons into the thylakoid lumen, creating a proton gradient across the thylakoid membrane.
• ATP synthesis: Protons flow back across the thylakoid membrane through ATP synthase, driving ATP synthesis.
• PSI: Excited electrons from PSI are used to reduce NADP+ to NADPH.

Calvin Cycle

• The Calvin cycle is a cyclic series of reactions that take place in the stroma of the chloroplasts.

• Carbon fixation: The process of incorporating carbon dioxide into an organic molecule.

• RuBP: Ribulose bisphosphate, a 5-carbon sugar that is the initial carbon acceptor in the Calvin cycle.

• Rubisco: The enzyme that catalyzes the reaction between RuBP and carbon dioxide, initiating the Calvin cycle.

• One turn of the Calvin cycle:

  • Consumes 1 CO2
  • Uses 3 ATP
  • Uses 2 NADPH
  • Produces 1 G3P (glyceraldehyde-3-phosphate)
  • Regenerates RuBP.

G3P Production

• It takes 6 turns of the Calvin cycle to produce one molecule of G3P, which is the basic building block for carbohydrates such as glucose.

Photorespiration

• Photorespiration is a process that occurs in plants under hot and dry conditions, when stomata close to prevent water loss.
• Rubisco, the enzyme involved in carbon fixation, can also bind to oxygen. When oxygen binds, the process results in the breakdown of organic molecules and the release of carbon dioxide, reducing efficiency of photosynthesis.

C3 Plants and Hot Conditions

• In C3 plants, photorespiration occurs under hot and dry conditions. Stomata close to reduce water loss, and the concentration of CO2 inside the leaf decreases. This leads to a higher oxygen concentration relative to carbon dioxide, favoring the oxygenase activity of Rubisco, and photorespiration occurs.

Consequences of Photorespiration

• Reduced photosynthetic efficiency: Photorespiration consumes energy and releases fixed carbon dioxide, reducing the overall productivity of photosynthesis.

Photosynthetic Adaptations

• C4 photosynthesis: In C4 plants, a special mechanism concentrates carbon dioxide near Rubisco, minimizing the amount of oxygen available to it and reducing photorespiration.

• CAM photosynthesis: In CAM plants, stomata open only at night, when temperatures are cooler. Carbon dioxide is fixed at night and stored as malate. During the day, the stomata close, and the malate is decarboxylated releasing CO2 to nourish the Calvin cycle.

Global Significance of Photosynthesis

• Food source: 50% of the carbohydrates produced by photosynthesis are estimated to be consumed by humans and other heterotrophs.

• Oxygen production: Photosynthesis provides almost all of the oxygen in the Earth's atmosphere. Oxygen is essential for respiration and many other biological processes.

Global Climate Change

• Greenhouse effect: Increased levels of greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, trap heat in the atmosphere, contributing to climate change.

• Impact on plants: Rising temperatures, changing precipitation patterns, and increased extreme weather events can negatively affect plant growth and species distribution.

Ozone Hole

• The ozone hole is a region of depleted ozone in the stratosphere over Antarctica.

• Chlorofluorocarbons (CFCs), formerly used in refrigerants and aerosols, were responsible for the depletion of ozone.

• Ozone (O3) protects life on Earth from harmful ultraviolet radiation.

• The Montreal Protocol, an international treaty, phased out the production of CFCs, leading to improvements in the ozone layer. However, it is estimated that full recovery of the ozone layer will take decades.